1. Field of the Invention
This invention relates to a hoisting device with a vertical motion compensation function, especially the hoisting device with the vertical motion compensation function eliminating effects by the heaving of a ship caused by waves with respect to an article hung from the ship through a wire.
2. Description of the Related Art
In the prevailing oceanographic observation, the sea area is grasped three-dimensionally based on the data of a vertical distribution and a horizontal distribution of a salinity concentration, a water temperature and a water depth measured at the same time. There is a CTD (conductivity temperature depth) observation instrument as an instrument for gathering the above data. The observation instrument is used for observing the CTD under a situation maintained in a constant depth, hung by a winch from a board of a ship to the sea. There is a case that measurements are performed, allowing the observation instrument to be raised or to be lowered at a constant speed. Furthermore, there are cases, for example, such that works are performed with a work robot held in a constant depth of the sea, and such that structures or the topography of the sea floor are observed with a camera hung from a board of a ship held in a constant depth by using a lifting device on the ship.
In such cases, when a hull is moved vertically by waves, an equipment such as the observation instrument in the water is also moved vertically in accordance with the vertical motion of the hull, as a result, accurate and speedy observations or works can not be performed. Therefore, it is required that the observation instrument or the equipment hung from the hull is allowed to be held in a constant depth or to be raised or lowered at a constant speed, and patent document 1 disclose a heaving-compensation type crane for the purpose.
FIG. 4 is a schematic view of a heaving-compensation type crane according to patent document 1. As shown in FIG. 4, in a crane system 2 installed on a hull 1, a wire 5 is payed out from a winch 3 through a crane boom 4, and an observation instrument 6 is hung at a tip of the wire 5. An accelerometer 7 is installed at a tip portion of the crane boom 4. The accelerometer 7 detects a vertical motion of the tip portion of the crane boom 4 caused by a vertical motion of the hull 1, and an output signal thereof is fed to a cylinder control unit of the crane system 2 not shown. Accordingly, when the hull 1 is moved vertically (heaving) by waves, the cylinder control unit offsets a vertical motion of the tip portion of the crane boom 4 by allowing a rod of a cylinder 8 to expand and contract to maintain the observation instrument 6 in the sea hung by the wire 5 in a constant depth.
However, the above crane system 2 has the following problems. Since the crane boom 4 composing the aforementioned crane system 2 is a large-sized heavy object, a large-sized cylinder is needed for allowing the crane boom 4 to move vertically and large energy is consumed, which increases costs. And further, because the crane boom 4 is the large-sized heavy object, it is difficult to move it vertically at high speed in accordance with the vertical motion caused by waves, as a result, the crane boom can not sufficiently follow up the vertical motion of the hull, which leads to a poor response.
Additionally, in a data observation by the CTD observation instrument, there is the case that a measurement is performed, allowing the instrument to be raised or to be lowered at a constant speed. The heaving-compensation type crane according to patent document 1 is not responsive to lift up or to lift down a lifting load at a constant speed when the full moves vertically by waves.